Hydrogel compression and polymer osmotic pressure

Controlling the elastic modulus of simple synthetic hydrogels like polyacrylamide is essential to their use in many areas of biotechnology, including tissue engineering, medical device development, and drug delivery applications. Indentation-based methods for measuring hydrogel elastic moduli are preferred over measurements in shear rheometers or tensile testing instruments when the freedom to choose sample volume and shape are restricted; contact lenses represent such an example. It is often believed that the local application of indentation loads will volumetrically compress hydrogels, increasing the sample's polymer concentration even when the applied pressure is less than the hydrogel's osmotic pressure. Here, we test this idea by volumetrically compressing polyacrylamide hydrogels of different compositions while measuring the degree of compression with increasing applied pressure. Our results reveal that at applied pressures below the hydrogel osmotic pressure, the gels exhibit only marginal compression, while above the osmotic pressure the gels compress as predicted by classical polymer physics theory. Combining measurements of osmotic pressure and polymer mesh size, we determine the scaling relationships between hydrogel composition, mesh size, and osmotic pressure. By demonstrating agreement between experiment and theory, we use our measurements to determine the Kuhn length of the individual polymer chains constituting the hydrogels.